1. Field of the Invention
The present invention relates to a water-jet pump (hereafter, also referred to simply as a jet pump), an impeller for the water-jet pump, and a boat including the water-jet pump.
2. Discussion of the Background
FIGS. 9A and 9B show a conventional-type jet pump. This known-type water-jet pump is provided with an impeller 3 installed in a cylindrical housing. The impeller 3, which is driven to revolve, includes a shaft body 1 and plural blades 2 that are formed on the outer circumference of the shaft body 1. In the impeller 3, a front portion 2f of each one of the blades 2 overlaps a rear portion 2r of the adjacent one of the blades 2, when viewed in the axial direction of the impeller 3. See, for example, FIGS. 3 to 5 of Japanese Patent Application Publication No. H9-99895. The contents of Japanese Patent Application Publication No. H9-99895 are incorporated herein by reference in their entirety.
In a jet pump of this kind, a water jet is produced by driving the impeller 3 to revolve, propelling the boat.
The impeller 3 shown in FIGS. 9A and 9B has a configuration to improve its pumping efficiency. Specifically, each of the blades 2 is formed so long in the rotating direction of the impeller 3 that the front portion 2f of each one of the blades 2 overlaps the rear portion 2r of the adjacent one of the blades 2 when viewed in the axial direction of the impeller 3. A region thus formed by the overlapping is referred to as an overlap region OL.
In the conventional jet pump shown in FIG. 9A, rear edges 2e of the blades 2 are formed extending in straight lines with respect to the respective radial directions towards a shaft center O when viewed in the axial direction of the impeller 3.
Now, suppose a case where an either stopped or slowly-travelling boat starts to advance at full speed and another case where a boat goes circling. In these cases, the pressure at the forward side of the impeller 3 decreases while the impeller 3 revolves a high speed. Here, the above-described blades 2 of the impeller 3 brings about a problem of vibrations of impeller 3 caused by the cavitation. The vibrations of the impeller 3 are transmitted by the jet pump, and generate vibrations of the boat body and annoying sounds.
To be more specific, in each overlap region OL formed by the front portion 2f of each one of the blades 2 and the rear portion 2r of the adjacent one of the blades 2, a high-pressure region in the front portion 2f of that one of the blades 2 interferes with a low-pressure region in the rear portion 2r of the adjacent one of the blades 2. The interference causes an abrupt pressure fluctuation, and the water flow is disturbed to a great degree. Accordingly, vibrations and annoying sounds are generated, and are then passed on to the boat body via the impeller 3 and the jet pump.